1. Field of the Invention
This invention relates generally to microwave phase shifters of electromagnetic energy and more particularly to electrically controlled phase shifters of microwave and millimeter wave signals.
2. Description of the Prior Art
Microwave or millimeter wave phase shifters are generally known and typically comprise ferrite type phase shifters located in waveguide transmission line circuits. A phase shifter is generally characterized by a two port RF transmission line where the phase of the output signal is varied with respect to the input signal by changing the field in which the ferrite is immersed. Phase shifts up to 360.degree. are obtainable in a relatively small structure.
More recently, an electrically controlled phase shifter has been developed which uses a transmission line fabricated from material which changes its permittivity by changing an applied DC electric field in which it is located. Such a device is shown and described, for example, in U.S. Pat. No. 5,032,805 issued to Frank J. Elmer et al on Jul. 16, 1991. The teachings of this patent are meant to be incorporated herein by reference. The device disclosed in the Elmer et al patent is constructed from a ceramic material, such as strontium-barium titanate, the permittivity of which changes with changes in applied electric field. The change in permittivity results in the change in the effective electrical length of the device, thus changing the delay or phase of an electromagnetic wave propagating through the device. Moreover, the device comprises an analog type of phase shifter requiring a voltage drive circuit having a variable voltage output to control the amount of phase shift provided.